Several publications are referenced in this application in order to more fully describe the state of the art to which this invention pertains. The disclosure of each of these publications is incorporated by reference herein.
Mismatch repair stabilizes the cellular genome by correcting DNA replication errors and by blocking recombination events between divergent DNA sequences. The mechanism responsible for strand-specific correction of mispaired bases has been highly conserved during evolution. Eukaryotic homologs of bacterial MutS and MutL, which are believed to play key roles in mismatch repair recognition and initiation of repair, have been identified in yeast and mammalian cells. Inactivation of genes encoding these activities results in large increases in spontaneous mutability, and in the case of humans and rodents, predisposition to tumor development.
Lynch syndrome or hereditary nonpolyposis colon cancer (HNPCC) is an autosomal dominant disease, which accounts for approximately 1-5% of all colorectal cancer cases. In this syndrome, colorectal tumors are frequently associated with extracolonic malignancies, such as cancers of the endometrium, stomach, ovary, brain, skin and urinary tract. Tumors from HNPCC patients harbor a genome-wide DNA replication/repair defect. Due to the lack of pathognomonic morphological or biomolecular markers, HNPCC has traditionally posed unique problems to clinicians and geneticists alike, both in terms of diagnosis and clinical management.
Recent breakthroughs in molecular biology have partially elucidated the pathogenic mechanism of this syndrome. Germline mutations in any one of five genes encoding proteins that participate in a specialized DNA mismatch repair system give rise to a predisposition for cancer development in HNPCC families. Patients affected by HNPCC carry these mutations in genes which are involved in DNA mismatch repair. The DNA mismatch repair mechanism contributes to mutational avoidance and genetic stability, thus performing a tumor suppressor function. Loss or inactivation of the wild type allele in somatic cells leads to a dramatic increase of the spontaneous mutation rate. This, in turn, results in the accumulation of mutations in other tumor suppressor genes and oncogenes, ultimately leading to neoplastic transformation.
Microsatellites are repeating sequences that are distributed throughout the human genome, most commonly (A)n/(T)n and (CA)n/(GT)n. Their function is unknown, but they are useful in genetic linkage studies because of their high degree of polymorphism and normally stable inheritance. Several of the genes responsible for HNPCC have been identified using analysis of mutation rate in DNA microsatellites. Mutations of mismatch repair genes can be detected in a subset of sporadic colonic and extracolonic cancers which exhibit variability in the length of microsatellite sequences. This variability is often referred to as microsatellite instability.
Investigators in the field (Peltomaki et al., (1993) Science 260:810-812) have discovered that most colorectal cancers from HNPCC patients show microsatellite instability. These studies revealed that the length of microsatellite DNA at different loci varies between tumor DNA and non-tumor DNA from the same patient. The phrase “replication error positive” (RER+) has been used to describe such tumors. It should be noted that only about 70% of HNPCC cases and only about 65% of sporadic tumors with microsatellite instability carry mutations in the known mismatch repair genes (hMSH2, hMLH1, hPMS2, hMSH6 and hPMS1) (Liu et al., (1996) Nature Medicine 2:169-174). The remaining 30-35% of the cases have an as yet unidentified mismatch repair genetic defect. Thus, there is a pressing need to identify the other active components in the DNA mismatch repair pathway, as mutations in these genes may result in an increased propensity for cancer.
The Fragile X or Martin Bell syndrome is the most common single recognized form of inherited mental retardation. Fifty percent of all X-linked mental retardation may be attributable to the Fragile X syndrome. The disorder is found in all ethnic groupings with a frequency of 0.3-1 per 1000 males and 0.2-0.6 per 1000 females. The full clinical syndrome, which is found in approximately 60% of affected males, consists of moderate mental retardation with an IQ typically in the range 35-50, elongated facies with large everted ears, and macroorchidism. This syndrome is unusual in that it is associated with the appearance of a fragile site on the long arm of the X chromosome at Xq27.3 (Sutherland, G. R., (1977) Science 197:256-266). This can be visualized cytogenetically in metaphase chromosomes prepared from lymphocytes of affected individuals which have been cultured under conditions of folate deficiency or thymidine stress. The study of the segregation of polymorphic markers within fragile X families has confirmed that the mutation lies in the same region of the X-chromosome as that exhibiting cytogenetic fragility.
There is an imbalance of penetrance of the phenotype associated with this syndrome in the different generations of kindreds in which the mutation is segregating. The likelihood of developing mental impairment depends on an individual's position in the pedigree. As the mutation progresses through the generations, the risk of mental impairment increases. These observations are not consistent with classical X linkage and are collectively known as the Sherman paradox. Hypotheses based on these observations have suggested that the mutation exists in two forms—a premutation and a full mutation form. Nonpenetrant individuals are said to carry a premutation chromosome, that is, a chromosome which has no abnormal phenotypic effect but which is capable of progressing to a fully penetrant mutation on passage through a female oogenesis.
Two alterations in the DNA at the fragile X site have been identified: abnormal amplification of a CpG-rich DNA sequence (a CpG island) and hypermethylation of such sequences. The molecular basis of the amplification is the expansion of a CGG triplet microsatellite into large arrays. In individuals expressing the full clinical phenotype, the DNA in this region becomes hypermethylated, leading to the transcriptional shut down of the gene FMR-1 (fragile X mental retardation 1) which is transcribed across this region. The clinical phenotype is likely caused by a loss of gene expression. It has been postulated that in Fragile X syndrome, expansion of the (CGG)n repeat from premutation to full mutation may be related to an aberrant (misdirected) DNA mismatch repair event. This may be favored by the transient lack of multiple methyl signals in the CGG repeat as well as in flanking single copy sequences during early stages of embryonal development. Similar to Fragile X syndrome, defective DNA mismatch repair may play a role in the expansion of triplet repeats associated with several disorders such as myotonic dystrophy, Huntington's disease, spino-cerebellar ataxias and Kennedy's disease.
The isolation of nucleic acids and proteins which, when mutated, give rise to these various disorders, enables the development of diagnostic and prognostic kits for assessing patients at risk. The biochemical characterization of the genes encoding the components of the DNA mismatch repair system may ultimately facilitate gene replacement therapies for use in the treatment of malignancy and other inherited genetic disorders.